Another 40 percent have thought about it, but that's as far as they got. Still, they're ahead of the 15 percent who haven't even considered what they might do about health-care costs once they retire.

The problem is drawing notice. A recent survey by AARP also found that 40 percent of over-50s have not managed even to begin to save for retired health-care costs—not a good sign.

While 88 percent of the respondents to the Ameriprise survey said that healthy lifestyle choices now will cut the amount they'll need to spend on health care in retirement, only 62 percent report that they're actively pursuing either a diet or an exercise program to try to cut those costs.

See also: CBO projects slower health spending rate

Of course, diet and exercise will only go so far, and deep down boomers know it: 78 percent say they're concerned about how healthy they'll be in retirement, and 26 percent say that they, or their spouses, have already been through a serious health event. Of those, 54 percent say it's affected their finances.

So what actions, if any, have they taken? About one-third have an advanced directive that expresses their wishes concerning medical care. A little less than a quarter (23 percent) are working longer, to keep their employer's health-care coverage longer. Only 21 percent have bought long-term care insurance, and only 17 percent have maxed out an employer-sponsored health savings account.

More people have thought about doing those things than have actually done them. About one-third of respondents are considering an advanced directive; about one-third are contemplating LTC insurance. Only 19 percent have thought about putting money into an HSA.

"Boomers understand that health care costs will be a significant expense in retirement, yet many haven't planned — or simply don't know how to plan — to fund these expenses," said Pat O'Connell, executive vice president, Ameriprise Financial, in a statement.

O'Connell added, "Fortunately, the national conversation around health care is prompting people to consider their health and the choices they make, as well as the need to build health-care expenses into their retirement and long-term financial plan. Pre-retirees have an opportunity to translate this awareness into tangible steps to help prepare for expected and unexpected health-care costs and protect the savings they've worked so hard to accumulate."